Frankly, over the years, I have been shocked at how poorly people handle departures. I have seen very senior people who are managing both teams and projects give two weeks notice, with zero indication that they were even looking. Some of those same people only days or a few weeks before have actually stated that they were committed and not looking. Some even just completed and signed off an annual review and compensation process.

I think point 1 works if the person has been with the company for a while and are successful and productive. While the employer is unlikely to walk a person out that comes to them openly and honestly, if they have been an underperformer for a while, the employer may not be open to having them stay around for an extended period while they look for something else. That said, if the employee is sure the the employer values them and their contributions, this is definitely the right approach. If you only have one foot out the door, this may spark a discussion on some changes that can be made to keep you. Employees would likely be surprised that employers don't always realize they aren't fully satisfied at work.

Points 3, 4 and 5 are spot on. WRT point 5, people often panic when someone, particularly if they are senior, quit. Departures allow an opportunity for other employees to step up and seize the opportunity left by a more senior person moving on, and it also allows the company to hire someone that is even better.

With all the discussion and reality about how frequently people change jobs nowadays, I continue to be surprised by how shocked and upset employees are when a colleague quits and at how poorly people handle resignations.

Lastly, if you feel you are of any value to the company, enough with the two weeks notice - you can offer that, but also offer to stay longer and be open to discussing an appropriate exit plan and date with your employer.